1. Field of the Invention
This invention is concerned with an air gun having a reciprocating shuttle valve for use in marine seismic exploration such as will be found in class 181/120.
2. Description of the Prior Art
The subject matter of this invention provides an improvement for the air guns disclosed in U.S. Pat. Nos. 4,180,139, 4,211,300 and a copending application Ser. No. 135,398, all of which are commonly owned by the assignee of this invention. U.S. Pat. No. 4,211,300 is incorporated herein by reference with respect to features that are common to that patent and to this disclosure.
U.S. Pat. No. 4,211,300 discloses an air gun that consists of an elongated cylindrical housing, closed at both ends by end plates, for containing a volume of compressed air and which forms a firing chamber. A radially-positioned air exhaust port is bored through the wall of the housing. A hollow cylindrical shuttle valve is mounted concentrically inside the housing. The shuttle valve is adapted to slide linearly back and forth, inside the housing, in alternate strokes. A radially-positioned sealing pad, having an orifice that is alignable with the exhaust port, is mounted in the wall of the shuttle. When the shuttle is positioned at either end of the housing, the pad blocks the exhaust port. When the shuttle is driven from one end of the housing to the other end, the orifice in the pad passes by the exhaust port and momentarily unblocks the exhaust port to impulsively release some of the compressed air from the firing chamber.
The shuttle valve is pneumatically driven from one end of the housing to the other. The ends of the hollow shuttle are closed by end faces. A space between the shuttle end faces and the respective end plates of the housing form shuttle-actuation control chambers. When the shuttle is at rest, an air bleed valve reduces the air pressure in the actuation control chambers to ambient. To fire the gun, an air diverter valve diverts some of the compressed air from the firing chamber to a shuttle-actuation control chamber at one end of the housing. The inrush of compressed air greatly exceeds the air leakage rate through the air bleed valve in that chamber; the shuttle valve is therefore pneumatically accelerated towards the opposite end of the housing. For the next stroke, a diverter valve at the other end of the gun diverts air from the firing chamber to the other actuation control chamber to drive the shuttle valve back to the first end of the housing. Thus, the shuttle is driven back and forth, within the housing, in alternate strokes. Each time the orifice in the sealing pad passes by the exhaust port, some of the compressed air is released from the firing chamber to generate an acoustic pulse.
As taught by the above-referenced patent, compressed air was admitted through a single opening directly into the main firing chamber. Some of the compressed air was then diverted from the firing chamber to one or the other of the shuttle-actuation control chambers to fire the gun. When the gun was fired, there was a considerable momentary pressure drop as the compressed air was released from the firing chamber through the exhaust port. The pressure drop in the firing chamber was unavoidably communicated to the actuation control chamber through the diverter valve. The reduced pressure was at times insufficient to drive the shuttle completely to the far end of the housing; the shuttle hung up in mid-stroke. Furthermore, when the ambient water pressure exceeded the air pressure drop, the gun became flooded.
Another problem involved shuttle-valve lubrication. Lubrication of the shuttle valve and the moving parts of the diverter valves depended on the oil vapor content of the compressed air. The oil vapor may be derived from piston-ring blow-by in the air compressor cylinders or oil vapor may be furnished by a vaporizer connected in the compressed air line. With but a single inlet for the compressed air, oil-vapor distribution became unbalanced. Accordingly, certain moving parts had a tendency to freeze-up due to lack of lubrication.